This invention relates to methods and apparatus for determining the polarity of a geophone, and more particularly, relates to methods and apparatus for determining the polarity of geophones mounted in seismic cables.
A conventional moving coil geophone consists of a coil of wire wound on a core and suspended by springs which allow the coil to move either up and down (for a vertical geophone) or from side to side (for a horizontal geophone) within the radial magnetic field of a center pole piece and a magnetically permeable case. Wires from the ends of the coil are connected to external terminals of the geophone, and for one convention one terminal is marked with a dot of paint or a plus sign to indicate a positive polarity. For this convention, positive voltage is produced on the plus terminal when the geophone coil is displaced downward (for a vertical geophone) with respect to the case, and the voltage is proportional to the relative velocity of the coil with respect to the case. Similarly, for one convention, a horizontal geophone will produce a positive voltage on its plus terminal when its coil is displaced to the right with respect to the case (when viewed from the front) and again the voltage is proportional to the relative velocity between the coil and case. It is also possible to combine a vertical and one or two perpendicular horizontal geophones in a single case or mounting to provide a multicomponent geophone. The term "geophone" without a modifier is used herein to mean a vertical, horizontal, and/or multicomponent geophone. Any mechanical impulse that is applied to the geophone along its sensing axis will normally generate a detectable voltage.
In seismic exploration the use of an array of geophones in a cable to record the ground motion at various survey locations is customary. The geophones may be connected in series, parallel, or a combination of series and parallel, depending upon acquisition factors. Correct polarity of each geophone is important so that the detector array response is the true sum of all the geophone elements. Detection of individual geophone polarities once assembled in a seismic cable, i.e. an array, is at best a difficult problem.
Traditionally, geophone polarity for each vertical geophone has been determined with the use of a zero center scale galvanometer (or a microammeter) in an enclosed facility used to assemble or repair geophone cables. A vertical geophone is positioned on its side and connected to the galvanometer. When the geophone is moved to its upright position, the galvanometer deflects either plus or minus A positive deflection of the galvanometer should occur if the geophone plus terminal is connected to the plus terminal of the galvanometer. If the positive geophone terminal is incorrectly marked, the galvanometer will have a negative deflection. Also, if the positive geophone terminal is incorrectly marked the marking must be changed to reflect the actual positive terminal. A horizontal geophone may be similarly checked and a multicomponent geophone ma have each component separately and individually checked in a similar manner.
Further, an oscilloscope may be used to replace the galvanometer and the transient displacement of the electron beam (i.e. output voltage of the geophone) indicates the polarity when a vertical geophone is moved upright from its side. Alternatively, a vertical geophone may be mechanically impulsed, i.e. tapped lightly on its top, and the output voltage observed to determine polarity. If a digital storage oscilloscope or other recording device is used, then the transient deflection can be stored for later analysis. However, the use of oscilloscopes in the field to check the polarities of geophones installed in cables is not very practical due to the size and bulk of an oscilloscope, as well as its need for power.
Both of these methods for determining the proper polarity of geophones require extreme care in handling, a lot of judgement, and attention to detail by the observer. In addition, the testing of geophones for polarity once installed in cables laid out for field acquisition is not practical using these methods.
These and other limitations and disadvantages of the prior art are overcome by the present invention, however, and methods and apparatus are provided for determining and/or checking a geophone for proper polarity.